This invention relates to a system for optically transmitting data as optical pulses through an optical fiber that serves as a data bus. Such a system will herein be referred to as an optical data transmission system. More particularly, this invention relates to an optical data transmission system comprising an automatic gain or threshold control facility known in the art. When the optical data transmission system comprises a central processing unit, the data may include commands generated by the central processing unit. This invention relates also to a transmitter, a receiver, and/or a data bus coupler for use in an optical data transmission system.
An optical data transmission system is excellent as a data transmission system particularly when it is inevitable to install a data bus through a location where a strong electric field is variable with time. This is because optical fibers are free from electromagnetic induction. An optical data transmission system comprising a central processing unit is therefore quite useful as a measuring and controlling system such as, for example, a centralized supervision and remote control system for collecting data from a number of devices in a remote power plant, industrial plants, office buildings, or a transport system and for controlling those devices.
As will later be described again with reference to a few of several figures of the accompanying drawing, an optical data transmission system usually comprises an optical fiber cable and a plurality of data bus couplers, each comprising a transmitter and a receiver. The transmitters and the receivers are coupled to the optical fiber cable. The transmitter and the receiver of one of the data bus couplers may be connected to a central processing unit, with the transmitters and the receivers of the other data bus couplers connected to the devices of the type exemplified above. The transmitter connected to the central processing unit sends commands and/or other data as optical pulses to the optical fiber cable. Responsive to the optical pulses, each receiver produces a received electrical signal and reproduces the commands and/or the data. One of the transmitters that is connected to the device specified by the commands with or without the data, converts data signals supplied from that device to optical pulses for transmission through the optical fiber cable. Supplied with the last-mentioned optical pulses, the receiver connected to the central processing unit reproduces the data sent back from the specified device.
The span between two adjacent data bus couplers may vary from a few meters to several kilometers. It is therefore very desirable that each receiver should comprise an automatic gain or threshold control circuit for shaping the received electrical signal into a sequence of correctly shaped pulses by carrying out peak detection of the received electrical signal.
The bit rate of the optical pulses may vary from zero (direct current) to the order of megabits per second. The peak detection fails to correctly shape the pulses whenever the bit rate becomes low. In this event, noise is amplified by the gain or threshold control circuit. Furthermore, not only are the pulses deformed but also errors appear in the pulse sequence when the automatic threshold control circuit is resorted to. The pulse distortion and errors are rendered more serious when the optical data transmission system comprises a greater number of data bus couplers.
An approach for removing the adverse effects caused on the automatic gain or threshold control when the bit rate becomes low, is therefore proposed by Hewlett-Packard Components, California, U.S.A., in pamphlets No. 5953-0375 "Fiber Optic Digital Receiver HFBR-2001" and No. 5953-0376 "Fiber Optic 100 Meter Digital Transmitter HFBR-1001" as tentative data June 1978. According to the proposal, a "refresh pulse" sequence is superposed on both high and low levels of the binary (two-level) data signals in the transmitter. The binary data signals are thereby converted to ternary (three-level) data signals in which pulses variable between high and medium levels are substituted for the high level of a long duration in the binary data signal and pulses variable between low and medium levels, for the low level of a long duration in the binary data signal.
The approach is effective in keeping the automatic gain or threshold control circuit in proper operation. The bit rate may vary from zero to as high as ten megabits per second. The ternary data signals, however, are ineffective in using the optical energy transmitted through the optical fiber cable. In other words, it becomes unavoidable to render the span between two adjacent data bus couplers short. This is necessary also for correct operation of the gain or threshold control circuit. Use of ternary data signals complicates the circuitry of the transmitter and the receiver. The ternary data signals become unbalanced when the electrooptical or the optoelectrical conversion characteristics are subjected to change either by the variation in the ambient temperature or as a result of the total time of use of an element for carrying out the conversion between the electrical and the optical pulses. Furthermore, the ternary pulses are subject to distortion because no consideration is given to instants of build up and down of the binary data pulses and the refresh pulses.